The invention proposes a friction disk, especially for a motor-vehicle dry clutch.
The invention more particularly proposes a friction disk, especially a dry-clutch friction disk for a motor vehicle.
In a known way, the friction disk includes a support web in the form of an annular washer, the outer peripheral portion of which is subdivided into radial blades which, by virtue of folds, feature a central fixing part which is connected to the central portion of the web by a tangential fold which forms the foot of the blade and which is perpendicular.
The tangential fold allows an axial offset of the central fixing part with respect to the general plane of the central portion of the web. Friction linings are fixed on either side of the outer peripheral portion, for example by riveting or by bonding. Each friction lining comprises an outer friction surface and an inner fixing surface, a fixing area of which is fixed onto at least one lateral fixing face of a central part of a radial blade which is offset axially towards the said friction lining.
Such a design is illustrated, for example, in the document EP-A-0.579.554 (U.S. Pat. No. 5,452,783).
When such a friction disk is used for transmission of the rotational movement of the engine of a motor vehicle to the wheels, the central portion of the web is linked to the primary shaft of a gearbox either directly or by way of elastic members.
When it is being used in a clutch device, the radial periphery of the friction disk, especially the friction linings, is clamped axially between a pressure plate and a reaction plate. The plates are driven in rotation by the engine flywheel. The axial offset of the central fixing part of the blades with respect to the general plane of the central portion of the web and the mechanism for linking to the clutch pedal make it possible to obtain the application of the axial load, from a zero value up to the rated value of this axial load which is characteristic of the setting of the clutch, during a part of the movement of the clutch pedal, which is the active clutch-engagement phase. During the active clutch-engagement phase, the two linings of the friction disk approach one another over a distance called progressiveness travel. The graphical representation of the axial force of the pressure plates on the friction as a function of the relative movement of two linings under this force is called progressiveness curve. At the end of the clutch-engagement phase, depending on the setting and the characteristics of the clutch, the axial offset of the central fixing part of the blades with respect to the general plane of the central portion of the web may be practically cancelled out.
During the clutch-engagement phase, the axial clamping load is transmitted to the friction surfaces of the lining by the surfaces of the pressure and reaction action plates which are face-to-face.
The axial offset of the central fixing part of the blades is not cancelled out during this operating phase. The friction lining therefore bear essentially on the lateral fixing faces of the blades. The distribution of the pressures over the friction surfaces of the lining is therefore not uniform.
Moreover, at the start of the active clutch-engagement phase, the rotational speeds of the engine flywheel and of the primary shaft of the gearbox are not homokinetic. This difference in rotation induces friction between the pressure plate and one of the linings, as well as between the reaction plate and the other lining.
The energy dissipated by the friction between the linings and the pressure and reaction plates, as well as the heating and the wear induced on the linings, are increasing functions of the pressure between the friction linings and the plates. Thus, during the clutch-engagement phase, the wear on the friction surfaces of the friction linings is therefore more rapid facing their fixing areas.
The areas of the fixing surfaces of the linings, which are not fixing areas and which are not in contact with the support web, are called free areas, and they transmit a lower pressure in the course of the active clutch-engagement phase and, consequently, wear out less rapidly.
Thus, the free areas of the linings are pushed back overall by the plates, and become thicker in step with the wear on the areas of the friction surfaces facing the fixing areas.
Moreover, the high temperatures and stresses, especially in the areas of the friction surfaces facing the fixing areas, cause permanent deformations, for example by the phenomenon of creep.
The cumulated effect of these deformations is called incrustation.
Incrustation is therefore manifested as an overthickness of friction material in the region of the free areas, protruding with respect to the plane passing through the inner fixing surfaces of the linings. This friction material of the free areas, protruding, comes into contact with the lateral faces of the blades which are opposite the lateral fixing faces, during the active clutch-engagement phase, before the progressiveness travel is terminated.
Whenever free areas are in contact with the peripheral part of the web, these free areas participate in the transmission of the axial force exerted by the pressure and reaction plates, at a level equivalent to the participation of the fixing areas.
In this case, if the differential in the rotational speeds between the two friction linings and the pressure and reaction plates is not zero, the speeds of wearing of the portions of the friction surfaces, on the one hand facing free areas in contact with the web, on the other hand facing fixing areas, are similar.
The greater the incrustation of the friction linings, the more the free areas rapidly come into contact with the peripheral portion of the web, in the course of the active clutch-engagement phase.
When the rotational speeds of the engine flywheel and of the primary shaft of the gearbox are homokinetic, there is no friction between the pressure and reaction plates, on the one hand, and the linings, on the other hand. The wearing of the linings does not change.
When the active clutch-engagement phase is terminated, if the differential in the rotational speeds between the two linings and the pressure plates is not zero, the clutch slips. The slipping of the clutch induces wear which diminishes the differences in thickness of the free areas between the linings until the thicknesses of these free areas become uniform.
When the linings are not incrusted, the progressiveness curve of the friction is a monotonic, increasing and continuous function. The slope of this curve grows with the relative approach of the two linings, up to a finite value.
When the linings are incrusted, we can distinguish two periods. The first period corresponds to the relative movement between two linings from the free state until the first, thickest free area comes into contact with the support web. The part of the progressiveness curve corresponding to this first period remains monotonic, increasing and continuous. The second period corresponds to the residual application of the axial load transmitted by the pressure plates. The complementary part of the progressiveness curve corresponding to this second period commences with a break point with a substantial augmentation of the slope. In general, the slope tends towards infinity towards the end of this second period. The progressiveness travel is reduced.
Incrustation therefore has a negative impact on the driving comfort of the vehicle during the clutch-engagement phase. This is because, during the passage from the first to the second period, the torque transmitted by the friction disk as a function of the axial force, that is to say of the axial movement of the pressure and reaction plates towards one another, varies abruptly.
The abrupt variation in transmission of the torque causes a jolt which impairs the driving comfort of the vehicle.
During the first period, the transmission of the torque is achieved in large measure by the areas of the outer friction surface of the linings face-to-face with the fixing areas.
During the second period, the transmission of the torque is achieved by practically all the outer friction surface of the linings.
The present invention makes it possible particularly to increase the duration of use of the friction disk during which incrustation has no negative impact on comfort, so as to increase the duration during which the driving comfort of the vehicle is optimal.
The invention thus makes it possible to even out the distribution of the pressures on the friction surfaces of the linings, which makes it possible, on the one hand, to reduce the speed of incrustation as well as the speed of wear and, on the other hand, to increase the performance of the clutch, that is to say for identical friction surfaces of the linings, the friction disk according to the invention exhibits an increased lifetime and makes it possible to transmit a higher torque.
To that end, the invention proposes a friction disk, especially a motor-vehicle dry-clutch friction disk, including a support web in the form of an annular washer comprising a central portion and a outer peripheral portion subdivided into radial blades featuring a central fixing part connected to the central portion of the web by a tangential fold which forms the foot of the blade and allows an axial offset of the central fixing part with respect to the general plane of the central portion of the web, and including friction linings on either side of the outer peripheral portion, each friction lining including an outer friction surface and an inner fixing surface a fixing area of which is fixed onto at least one lateral fixing face of a central part of a radial blade which is offset axially towards the said friction lining, characterised in that an inner surface of a friction lining includes a reduced-thickness area, or thinned area, in an area separate from the fixing areas, so as locally to reduce the axial thickness of the friction lining, and in that at least one reinforcing element extends facing at least one part of a reduced-thickness area of the friction lining in such a way as to promote the uniform distribution of the pressures over the friction surface of the lining when it is stressed axially by a complementary plate.
By virtue of the invention, the quantity of friction material is reduced, while having good mechanical behaviour of the friction linings by virtue of the reinforcing element, such that the solution is economical.
According to other characteristics of the invention:
for a good distribution of the pressures and good parallelism of the friction linings, at least one of the radial blades features, in addition to the central part, at least one peripheral bearing-surface part a lateral bearing-surface face of which comes into abutment on a bearing-surface area of the inner surface of a lining which is opposite the lining on the inner surface of which is fixed the lateral fixing face of the central part of the radial blade;
the reinforcing element extends at least partially face-to-face with a lateral bearing-surface face of a peripheral part of a radial blade;
at least one reinforcing element extends substantially face-to-face with the inner periphery of the inner surface of at least one friction lining;
several reinforcing elements arranged between the inner surface of a lining and the web of the support are linked together so as to constitute a single element;
the single element includes a ring, the inner diameter of which corresponds substantially to the inner diameter of the lining facing which it extends, and branches which extend radially from the outer diameter of the ring towards the outer periphery of the lining;
at least one of the branches extends face-to-face with a lateral bearing-surface face of a peripheral part of a radial blade;
the reinforcing element includes a reinforcing sector bearing against at least one part of the reduced-thickness area, and at least one support lug which extends axially from the reinforcing sector towards the web against which it bears so as to enhance the uniformity of the distribution of the pressures over the friction surface of the friction lining;
at least one support lug bears on the lateral face opposite the lateral fixing face of the facing blade;
at least one support lug bears on the facing lateral face of the central portion of the radial web;
the support lug is produced from an elastic material so that it undergoes an axial deformation which is a function of the axial force exerted, by a plate, on the friction surface of the facing lining;
at least one support lug is obtained by cutting-out and/or folding of the reinforcing element to which it belongs;
at least one of the linings consists of blocks or tiles delimited by edges of overall radial orientation;
at least one of the linings consists of a continuous ring.
The invention also proposes a method of manufacturing a friction lining, for a motor-vehicle clutch friction disk as described above, characterised in that it includes a stage of firing the friction material of the friction lining in a mould the area of which which is in contact with the fixing surface of the lining includes at least one boss, so as to produce at least one reduced-thickness area or thinned area.
According to other characteristics of the method:
the friction material is injected into the mould;
it includes a stage of machining the fixing face of the friction lining so as to produce at least one reduced-thickness area or thinned area;
at least one reinforcing element is fixed to an element of the friction disk;
at least one reinforcing element is fixed to the support web;
at least one reinforcing element is fixed to the friction lining;
the friction lining is produced by overmoulding around a reinforcing element;
at least one reinforcing element is bonded onto an element of the friction disk;
at least one reinforcing element is riveted onto an element of the friction disk;
at least one of the linings is bonded onto a fixing surface of a radial blade of the support web.